Packet Traversal, Tables and Chains

A packet goes through several different routines and there are hooks available that allow you to modify the behavior.

There are primarily 3 different tables which allow you to define rules and where those rules get applied depends on the chain.

Consider the following diagram showing packet traversal flow:

According to above diagram, a packet goes through following chains:

PREROUTING: A packet goes through this chain before any routing decision is made.

FORWARD: If the message is not intended for local machine; it can be either dropped or routed to other machine based on the policy.

INPUT: If the packet is targeted for local machine, it passes through the INPUT chain.

OUTPUT: Any packet originating from local machine goes through the OUTPUT chain.

POSTROUTING: All outbound packet goes through post routing state.

Let’s go though the packet flow. When a new packet arrives:

It first goes through the PREROUTING chain; where you can change the TCP headers, setup nat rules, etc; before passing it on for the Routing. Routing decides if the packet is targeted for current system based on IP headers.

If it is not meant for the current system, packet passes through the FORWARD chain where one can decide to either forward the packet or drop it based on the set policy. If the forwarding is allowed, the packet is passed on to the POSTROUTING chain, otherwise it’s dropped or rejected.

If the packet is meant for the current system, it is passed to the INPUT chain where packets can be filtered, header can be modified, etc and once rules in this chain are applied, packet is passed on to the local machine for processing.

Any new packet from local machine passes through OUTPUT chain, where packet can be modified before the routing decision is made. Once the routing decision is made, packet goes through the POSTROUTING chain.

POSTROUTING change can modify the headers, filter the packets before packet is finally put onto the network.

I loosely talked about filters, modifying headers during the traversal flow of packet. To make it more concrete, let’s look at what tables are available and how can they be used. As mentioned above, there are primarily 3 tables available where you can define you rules and these rules are executed at different chains. These three tables are: nat, mangle and filter.

Not all tables are available in all chains, as you can see filter table is not available in PREROUTING and POSTROUTING chains.

So a rules needs to have a chain and a table. For example, an iptable rule looks something like this:

filter: This table, as the name suggests, filters the packets based on the rules. This is the default table and will be used if a table is not specified. It is only present in INPUT, OUTPUT and FORWARD chain. Based on target value, packets can be dropped, accepted, and returned.

nat: This allows altering the packets destination or source address. This is normally used to modify source and destination headers to allow internal ips to talk to outside networks. Normal use-case involves changing the destination IP address of the packet, also known as DNAT, in the PREROUTING chain to that of local machine so that routing can route it to INPUT chain. And conversely, changing the source address of the packet, also known as SNAT, in the POSTROUTING chain to that of the router or an ip known to destination routing.

mangle: Mangle can be used to modify certain properties of the packet like TOS, TTL, MTU, etc.

Targets and Jumps (-j flag)

Each rule has a predicate and a possible action which is called a target. For a detailed list of possible target and jump values, refer: iptables - targets and Jumps. Some of the most common ones are:

ACCEPT: This can be used to allow traffic based on the other specifications in the rule.

DROP: This can be used to drop the messages. No further processing is done; and no further information will be sent back to sender.

REJECT: This is similar to DROP but it also sends back an error message back to host sending the packet.

SNAT: It allows one to modify source ip address in the packet. It’s only valid in POSTROUTING chain and nat table.

DNAT: It allows one to modify destination ip address in the packet. It’s only valid in PREROUTING chaing and nat table.

MASQUERADE: This is same as SNAT but it doesn’t require a --to-source option. I.e., this is preferable if you don’t know the external ip during the rule creation, as it may be retrieved later using DHCP.

Some CLI examples

This post was an attempt to understand basics of iptables, rather than a specific example. There are some good examples online like this nat tutorial

To drop traffic from a certain IP address

We can use filter table (which is the default) and add the rule in the INPUT chain.